Cyclic transition to turbulence in rigid abdominal aortic aneurysm models

Abstract

The hydrodynamic stability of cyclic ows inside rigid abdominal aortic aneurysm (AAA) models was investigated. Rectiÿed sine waveforms were used to simulate aortic ow conditions (Re mean = 1600-2100 and = 7:2-12.2). Depending on the bulge geometry (D=d and L=d ratios), AAA ows can be broadly classiÿed into three regimes, namely types A, B and C, respectively. While type A has no vortex formation, type B and C have distinctive laminar vortical structures that are very di erent from one another. The type of ow regimes would also determine where and when the transition to turbulence would occur and the portion of the cycle at which the ow remains turbulent in the bulge. The stability characteristics of types B and C are obtained from the linear stability analysis performed on the unsteady velocity proÿles measured at di erent phases of a cycle. Based on the linear stability analyses, instability is found to initiate in the bulge for types B and C through the formation of vortical structures. Instability grows progressively during the acceleration phase and transition to turbulence in the bulge occurs shortly after the commencement of the deceleration phase in all cases investigated. The mechanisms of transition to turbulence for types B and C are discussed. Although transition to turbulence appears in all the cases investigated here, fully laminar ows in types B and C are predicted to exist by the linear stability theory under extreme ow conditions. Finally, the in vivo biological implications of the in vitro results were discussed.

Dynamics of pulsatile flow through model abdominal aortic aneurysms

“…Moreover, our results suggest that physical activity is harmful for a patient with an abdominal aortic aneurysm; the more-localized or well-developed aneurysms again being the most pathological. Yip & Yu (2001) and Salsac et al (2006) mention that during part of the cardiac cycle the flow in an aneurysm may become weakly turbulent. Since this repetitive occurrence of disturbed flow conditions seems especially harmful, we have investigated the hydrodynamic stability of pulsatile flows through a model aneurysm.…”

“…What has been established is that during part of the cardiac cycle the blood flow in the abdominal aorta becomes weakly turbulent during exercise, but it remains laminar during rest conditions (Les et al 2010). A local dilatation of the abdominal aorta may be expected to promote instability of the blood flow, and it is not unlikely that flashes of turbulence may occur during part of the cardiac cycle even during rest conditions (Yip & Yu 2001). This is believed to be beneficial, on the grounds that the presence of turbulence reduces the size of regions of flow stasis and the existence of a correlation between the presence of such regions and thrombus formation (Reininger et al 1994;Vorp et al 2001;Salsac et al 2004).…”

Cyclic flow characteristics in an idealized asymmetric abdominal aortic aneurysm model

“…Moreover, further experimental investigations by Yip and Yu [7] have shown that the onset of turbulence had actually taken place at the distal neck for a large AAA model (D/d =4) at the beginning of the deceleration phase of the ow cycle. Flow at the distal neck would remain turbulent for about two-fths of the decelerating phase.…”

Computational analysis of type II endoleaks in a stented abdominal aortic aneurysm model

“…Under pathological conditions, the peak Reynolds number may indicate local turbulence. Thus, more sophisticated simulation methods may be needed (Egelhoff et al, 1999;Yip et al, 2001). It should be noted that the significance of type II endoleak still remains controversial clinically.…”